Transmit leakage cancellation in a wide bandwidth distributed antenna system

ABSTRACT

A system and methods for cancelling transmission leakage signals in a wide bandwidth Distributed Antenna System (“DAS”) having remote units is disclosed. An internal cancellation circuit within the remote unit is employed to reduce the transmitted leakage signals by generating a cancellation signal. This cancellation signal is added to the received signal to cancel the transmission leakage signal in the receiving signal path. A pilot signal generation circuit is employed to optimize the cancellation circuit operating parameters. The frequency of the pilot signal is swept over a range to determine the pilot frequency having the highest electromagnetic coupling. The amplitude and phase of the cancellation signal is then optimized to minimize the level of transmission leakage in the receiving transmission path.

RELATED APPLICATION INFORMATION

The present application claims priority under 35 U.S.C. Section 119(e)to U.S. Provisional Patent Application Ser. No. 61/377,065 filed Aug.25, 2010, the disclosure of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to methods and systems relatedto wireless telephony. More particularly, the invention relates totransmission leakage cancellation systems and methods for remote units.

2. Description of the Prior Art and Related Background Information

Modern wireless telephone systems often employ Distributed AntennaSystems (“DAS”) throughout a region having multiple remote units. Eachof the remote units typically has a transmission and receiving antennawhich are physically separated in order to isolate the antennas. Inother applications, the antennas may not be sufficiently isolated suchthat the signal transmitted from the transmitting antenna appears on thereceiving antenna as a transmission leakage signal. This transmissionleakage signal detrimentally affects the performance of remote units.

Accordingly, a need exists to improve the performance of remote units.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a distributed antennasystem (“DAS”) comprising a plurality of wireless remote units coupledto one or more base stations. The wireless remote units comprise a firstsignal path for providing a transmission signal, a transmitting antennafor outputting the transmission signal, a receiving antenna forreceiving a received signal, a second signal path for providing thereceived signal, and a cancellation circuit coupled to the first signalpath and the second signal path, where the cancellation circuit includesmeans for providing a cancellation signal to the second signal path forsubstantially cancelling a transmission leakage signal in the receivedsignal.

In a preferred embodiment, the cancellation circuit preferably comprisesa delay circuit configured for matching a delay associated with thefirst signal path, the transmitting antenna, the receiving antenna, andthe second signal path. The means for providing a cancellation signalpreferably comprises a vector modulator configured for substantiallycancelling a transmission leakage signal in the received signal byadjusting the amplitude and phase of the cancellation signal. Thecancellation circuit preferably further comprises a bandpass filterhaving a cancellation bandwidth substantially less than the bandwidth ofthe DAS. The cancellation bandwidth preferably has a bandwidth in therange of approximately 60 Megahertz to approximately 200 Megahertz.

In another aspect the present invention provides a distributed antennasystem (“DAS”) including a plurality of wireless remote units. Eachwireless remote unit comprising a first signal path for providing atransmission signal, a transmitting antenna for outputting thetransmission signal, a receiving antenna for receiving a receivedsignal, a second signal path for providing the received signal, a pilotsignal generation circuit coupled to the first signal path and thesecond signal path, where the pilot signal generation circuit isconfigured for generating a pilot signal in the first signal path, and adetector for detecting the pilot signal in the second signal path. Thepilot signal generation circuit comprises a first bandpass filter havinga first bandwidth. Each wireless remote unit further comprises acancellation circuit coupled to the first signal path and the secondsignal path, where the cancellation circuit is controlled responsive tothe detected pilot signal for providing a cancellation signal to thesecond signal path for substantially cancelling a transmission leakagesignal in the received signal.

In a preferred embodiment the wireless remote units preferably furthercomprise a first amplifier in the first signal path and a secondamplifier in the second signal path. The pilot signal generation circuitis preferably coupled to the input of the first amplifier and is coupledto the output of the second amplifier and the cancellation circuit iscoupled to the output of the first amplifier and the input of the secondamplifier. The cancellation circuit preferably further comprises acancellation circuit bandpass filter coupled to the first signal path, adelay circuit coupled to the bandpass filter, and a vector modulatorcoupled to the delay circuit, where the vector modulator is configuredfor substantially cancelling a transmission leakage signal in thereceived signal by adjusting the amplitude and phase of the cancellationsignal, and an additive coupler coupled to the output of the vectormodulator and the second signal path. The pilot signal generationcircuit preferably comprises a controller configured for controlling thevector modulator based on the detected pilot signal in the second signalpath. The pilot signal generation circuit preferably further comprises alimiter configured for limiting overall gain of the pilot signalgeneration circuit, the first amplifier, the cancellation circuit, andthe second amplifier, and a phase shifter configured for shifting thefrequency of the pilot signal. The pilot signal generation circuitpreferably further comprises an absorptive switch configured fordisabling the pilot signal generation circuit.

The wireless remote units preferably further comprise a second pilotsignal generation circuit configured for providing a second pilot signalin the first signal path. The second pilot signal generation circuitpreferably comprises a second bandpass filter having a second bandwidthdiffering from the first bandwidth of the first bandpass filter. Thesecond pilot signal generation circuit is preferably coupled to theinput of the first amplifier and is coupled to the output of the secondamplifier The wireless remote units further comprise a second detectorfor detecting the second pilot signal in the second signal path and asecond cancellation circuit configured for providing a secondcancellation signal to the second signal path, where the secondcancellation circuit is coupled to the output of the first amplifier andthe input of the second amplifier. The pilot signal generation circuitpreferably further comprises a limiter configured for limiting gain ofthe pilot signal generation circuit, the first amplifier, thecancellation circuit, and the second amplifier, and a phase shifterconfigured for shifting the phase of the pilot signal. The second pilotsignal generation circuit preferably further comprises a second limiterconfigured for limiting gain of the second pilot signal generationcircuit, the first amplifier, the second cancellation circuit, and thesecond amplifier, and a second phase shifter configured for shifting thephase of the second pilot signal. The wireless remote units preferablyfurther comprises a second cancellation circuit configured for providinga second cancellation signal to the second signal path, where the secondcancellation circuit is coupled to the output of the first amplifier andthe input of the second amplifier. The pilot signal generation circuitis preferably further configured for generating a second pilot signal inthe first signal path.

In another aspect, the present invention provides a method fortransmission leakage cancellation in a wireless remote unit in adistributed antenna system (“DAS”), the remote unit having a firstsignal path coupled to a transmission antenna and a second signal pathcoupled to a receiving antenna. The method comprises generating a pilotsignal, providing the pilot signal to a transmission antenna, generatinga cancellation signal, receiving the received signal having atransmission leakage signal in a receiving antenna, providing thereceived signal to a second signal path, and cancelling the transmissionleakage signal by adding the cancellation signal with the receivedsignal.

In a preferred embodiment the method further comprises filtering thepilot signal with a bandpass filter having a first bandwidth andfiltering the cancellation signal with a second bandpass filter havingthe first bandwidth. Cancelling the transmission leakage signalpreferably comprises detecting a residual pilot signal after thecancellation signal has been added to the received signal, and adjustingan amplitude and phase of the cancellation signal based on the detectedresidual pilot signal. The method preferably further comprises shiftingthe frequency of the pilot signal to find a pilot frequency with ahighest RF coupling, and adjusting the amplitude and phase of thecancellation signal to minimize the detected residual pilot signal.Adjusting the amplitude and phase of the cancellation signal preferablyfurther comprises performing a two-dimensional descent-based search.Adjusting the amplitude and phase of the cancellation signal preferablyfurther comprises determining the optimized amplitude and phaseparameters.

Further features and aspects of the invention are set out in thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a Distributed Antenna System (“DAS”)having multiple remote units.

FIG. 2 is a schematic drawing of an exemplary remote unit depicted inFIG. 1.

FIG. 3 is a schematic drawing of a cancellation circuit employed by aremote unit in an improved DAS in accordance with the present invention.

FIG. 4 is a schematic drawing of a remote unit having a pilot signalgeneration circuit and a cancellation circuit.

FIG. 5 is a schematic drawing of a remote unit for cancellingtransmission leakage signals for two disjoint frequency bandwidths in anembodiment.

FIG. 6 is a schematic drawing of a remote unit for cancellingtransmission leakage signals for two disjoint frequency bandwidths inanother embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide systems and methods forcancelling transmission leakage signals in remote units in a DistributedAntenna System (“DAS”) in which the receiving antenna iselectromagnetically coupled to the transmitting antenna. An internalcancellation circuit within the remote unit is employed to reduce thetransmitted leakage signals. The cancellation circuit is coupled to thetransmission signal path and comprises a bandpass filter, a delaycircuit, and a vector modulator to generate a cancellation signal. Thiscancellation signal is then added to the received signal to cancel thetransmission leakage signal in the receiving signal path. A pilot signalgeneration circuit may be used to optimize the cancellation circuitoperating parameters. The pilot signal generation circuit introduces apilot signal in the transmission signal path, which is then transmittedand received by the receiving antenna. The pilot signal generationcircuit comprises one or more bandpass filters, a detector andcontroller unit, a limiter, a phase shifter, and an absorptive switch. Adetector and controller unit in the pilot signal generation circuitmeasures the residual pilot signal, and then controls the phase shifterto sweep the frequency of the pilot signal over a range to determine thefrequency having the highest electromagnetic coupling. The detector andcontroller unit controls the amplitude and phase of the cancellationsignal to minimize the level of transmission leakage in the receivingsignal path.

Wide bandwidth DAS systems are available that distribute signals alongseparate transmit and receive pathways. An exemplary wide bandwidth DAS100 is shown in FIG. 1. The transmit signal from a base station (BTS)such as BTS 110 and 112 is distributed throughout a building, using aprimary hub 130, to several remote units such as remote units 150 and152 that each have a small coverage area. BTS 110 and 112 may be coupledto the primary hub 130 using coaxial cables 120 and 122, and the primaryhub 130 may be coupled to the remote units 150 and 152 and a secondaryhub 132 using fiber optic links 140, 142, and 144 for example. Remoteunit 150 may be coupled to the transmission antenna 170 and thereceiving antenna 180 using coaxial cables 160 and 162 in an embodiment.Likewise, remote unit 152 is coupled to the transmission antenna 172 andthe receiving antenna 182 using coaxial cables 164 and 166. The receivesignal going to BTS 110 and 112 is obtained by combining signals fromthe remote units 150 and 152 using the primary hub 130. The coverage ofthe entire building is ensured by installing a sufficient number ofremote units placed throughout the structure. A secondary hub 132 may beneeded to further distribute the signals if the DAS network contains alarge number of remote units.

Consider an exemplary remote unit as shown in FIG. 2 which amplifiesboth the transmit and receive signals. Embodiments of the remote unitsdescribed herein such as remote unit 150 may have a first signal pathfor providing a transmission signal and a second signal path forproviding a received signal. The transmission signal path (i.e., thefirst signal path) comprises an optical-to-RF (“Radio Frequency”)converter 205 which receives signals from hub 130 through fiber opticlink 140 a. The RF signal is then amplified by amplifier 210. Theamplified signal is then fed into variable voltage attenuator (“VVA”)215. The output of WA 215 is coupled to the input of power amplifier(“PA”) 220 which provides the transmission signal to the transmissionantenna using coaxial cable 160. The receiving signal path (i.e., thesecond signal path) comprises a low noise amplifier (“LNA”) 235 whichreceives signals from the receiving antenna via coaxial cable 162. Theoutput of LNA 235 is coupled to WA 240, and is then amplified byamplifier 245. The output of amplifier 245 is converted into opticalsignals by RF-to-optical converter 250 which transmits the receivedsignals to the hub 130 using fiber optic link 140 b.

The remote unit employs WAs 215 and 240 on both RF paths to compensatefor the level shifting done within the fiber optic links 140 a and 140 bwhich connect the remote unit 150 and hub 130. There are no duplexfilters to separate the transmit and receive bands within the remoteunit 150 or hub 130. The isolation between the transmit and receive pathis achieved by separating the transmit (“Tx”) 170 and receive (“Rx”) 180antennas. The level of isolation is assumed to be 40 dB.

Consider the case where the isolation between the Tx and Rx antennas isnot sufficient. In this scenario, part of the Tx signal can appear inthe Rx signal path due to coupling between the Tx and Rx antennas. Thisinterference is not seen by the basestation such as BTS 110 and 112because the interference is rejected by the BTS's duplexer. However, theenergy of the Tx signal on the Rx path detrimentally consumes thedynamic range of both the LNA 235 and the fiber optic link 140 b. Inother words, the leaked Tx signal acts as a blocker within the Rx chain.

It is therefore desirable to cancel the Tx signal coupled into the Rxpath. Cancellation is performed over a narrow bandwidth compared to thewide bandwidth of the DAS system. FIG. 3 depicts an exemplary circuitemployed in the remote units (150, 152) in an improved DAS in accordancewith the present invention, illustrating the transmission signal path,received signal path, and a cancellation circuit. FIG. 3 alsoillustrates the RF coupling between the Tx antenna 320 and the Rxantenna 360.

Cancellation of the Tx leakage is achieved using an internalfeed-through cancellation circuit path from the Tx and Rx signal pathscontrolled by adjustable gain (amplitude and phase).The Tx signal path(i.e., the first signal path) comprises a WA 215 which receives thetransmission signal 300 from the output of an amplifier 210, a PA 220,and a Tx antenna 320. The Rx signal path (i.e., the second signal path)comprises an Rx antenna 360, an additive coupler 370 which sums thereceived signal with a cancellation signal discussed below. Theresultant signal is then fed to LNA 235 and then to WA 240. The receivedsignal 385 is sent to the input of amplifier 245.

The internal cancellation circuit path is coupled to the output of Tx PA220 and comprises a bandpass filter 335, a delay circuit 340, and avector modulator (“VM”) 345 to generate a cancellation signal. Thecancellation signal is fed to an input of the additive coupler 370 whichadds the received signal with the cancellation signal. The sum of thesignals is amplified by Rx LNA 235. A bandpass filter 335, having afrequency response denoted by H₁(ω), is used to limit the cancellationbandwidth (“BW”). In general, a cancellation bandwidth of 60 MHz to 200MHz could be expected. A delay circuit 340 is provided within thecancellation path so that the circuit delay matches the delay associatedwith the path formed by the feedlines connecting to the antennas and theRF coupling between antennas. A VM 345 is used to adjust the amplitudeand phase of the cancellation signal so that the Tx leakage within thecancellation BW is cancelled as much as possible.

It is desirable to tune the operating parameters of the VMautomatically. FIG. 4 illustrates an embodiment in which a pilot signalgeneration circuit is employed that first generates a pilot signal, andthen measures the power of the residual pilot signal after cancellation.The Tx signal path (i.e., the first signal path) comprises a VVA 215which receives the transmission signal 300 from the output of anamplifier 210, an additive coupler 418, a PA 220, and a Tx antenna 320.The Rx signal path (i.e., the second signal path) comprises an Rxantenna 360, an additive coupler 370, a LNA 235, and a VVA 240 whichgenerates a received signal 385 that is sent to the input of amplifier245. The pilot signal generation circuit comprises a bandpass filter 404coupled to the output of the LNA 235, a detector and controller unit408, a limiter 410, a phase shifter 412, an absorptive switch 414, and abandpass filter 416. The output of the bandpass filter 416 is fed intoadditive coupler 418 which adds the pilot signal with the signalgenerated by WA 215. In this non limiting example, the detector andcontroller unit 408 performs the functions of detecting and controlling,however, it shall be understood that these functions may be performed bytwo or more separate devices in an embodiment.

Nonlinear positive feedback through the limiter 410 results in a boundedoscillation if two conditions are met. The first condition, referred toherein as the “amplitude condition,” requires that the feedback loopgain of the pilot signal generation circuit (which includes limiter410), the PA 220, the parallel paths of the cancellation circuit and RFcoupling between antennas 320 and 360, and the LNA 235 has a gain ofunity. If the gain of the remainder of the loop is high enough thatgenerated signal is clipped by the limiter, then the amplitude conditionis met.

The second condition, referred to herein as the “phase condition,”requires that the phase around the feedback loop must be zero: that is,

2·π·n=ω _(pilot) ·T _(loop)+φ₀

where n is an integer, ω_(pilot) is the pilot frequency, T₁₀₀ is thedelay around the feedback loop, and φ₀ is a phase shift which can beadjusted using the phase shifter 412 in the pilot signal generationcircuitry. It can be seen that the frequency of the pilot ω_(pilot) willchange to compensate for the phase shift φ₀, ensuring that the phasecondition is met and an oscillation is present at some frequency. Notethat the oscillation is generated initially from circuit noise andmaintained by the regenerative effect of the positive feedback loop.

The pilot signal generation circuit contains one or more filters such asfilters 404 and 416, a limiter 410, a phase shifter 412, and anabsorptive switch 416. A detector and controller unit 408 is also neededto measure the residual pilot, which is an estimate of the Tx leakagealong the RF coupled path not cancelled by the cancellation path. Thedetector and controller unit 408 may control the VM 345, the phaseshifter 412, and the absorptive switch 414 through control lines 451,452, and 453 respectively. The frequency response of the filters in thegenerator is selected typically to match that of the filter 335 in thecancellation circuit H₁(ω). If the filters are different, the filter inthe generation circuit should have a narrower bandwidth that fallswithin the bandwidth of the filter 335 of the cancellation circuit.

The limiter 410 within the pilot signal generation circuit acts as anautomatic level controller for the feedback loop. Excess loop gain fromelsewhere in the feedback loop is removed by the clipping associatedwith the limiter 410.

The detector and controller unit 408 is used to measure the residualpilot signal. The VM 345 of the cancellation circuit path is adjusted bythe detector and controller unit 408 based on the power level detected.The VM 345 is considered properly tuned when the detected power level isminimized.

Assuming that the VM 345 of the cancellation circuit is set to a zeromagnitude (turned off), the nonlinear positive feedback will naturallyfind the frequency within the bandwidth of H₁(ω) that has the highest RFcoupling, ω_(RF), amongst those frequencies that meet the phasecondition. In order to find the maximum coupling within H_(i)(ω), thedetector and controller unit 408 sends control signals to the phaseshifter 412 within the pilot signal generation circuit to sweep thephase thus moving the pilot frequency over a small range. One may firstperform a search, such as a descent-based search, of the phase shifterrange to find the pilot frequency with the highest RF coupling using thedetector and controller unit 408 to control the phase shifter 412. Thecancellation circuit can then be tuned to find the VM 345 setting (gainand phase, or I and Q settings) that minimizes the detected pilot power.This can be performed using a two-dimensional descent-based search.

Detector and controller unit 408 must be located before the limiter 410to measure the residual pilot signal. However, other components may belocated in different places than shown in FIG. 4. For example, phaseshifter 412 may be on either side of limiter 410. Also note that one ofthe filters in the pilot signal generation circuit, such as filters 404or 416, can be removed if needed. The first filter 404 before thedetector and controller unit 408 tends to reduce the variance of themeasured pilot residual because some of the signals from mobiles presentin the coverage area of the remote unit will be filtered and attenuated.The filter 416 after the limiter 410 is used to remove harmonics of thepilot frequency. Harmonics of the pilot can be problematic if they fallin the BTS's receive band, so it is preferred that they be attenuatedusing a filter. However, only one of the two filters is needed for theoscillation to be generated properly.

The pilot signal generation circuit is used typically as part of acalibration procedure. The pilot signal generation circuit is disabledusing an absorptive switch 414 when the remote unit is operating as partof the DAS network. Absorptive switch 414 may be controlled by thedetector and controller unit 408 through control line 453 or by anexternal signal controlling DAS remote unit. However, it is possible tocalibrate a single remote unit while the remaining remote units withinthe DAS network are operating. In such cases, WA 215 on the Tx path andWA 240 on the Rx path of the remote unit being calibrated should be setto their maximum attenuation to avoid interfering with the network.

Several cancellation paths with disjoint frequency bandwidths can beincluded to perform Tx leakage cancellation for more bands. A pilotsignal generation circuit is needed for each cancellation circuit. Twopossible implementations are shown in FIG. 5 and FIG. 6.

FIG. 5 illustrates a system having a second cancellation circuit and asecond pilot signal generation circuit. The Tx signal path (i.e., thefirst signal path) comprises a VVA 215 which receives the transmissionsignal 300 from the output of an amplifier 210, additive couplers 418and 518, a PA 220, and a Tx antenna 320. The Rx signal path (i.e., thesecond signal path) comprises an Rx antenna 360, additive couplers 370and 570, a LNA 235, and a WA 240 which generates a received signal 385that is sent to the input of amplifier 245. The second cancellationcircuit is coupled to the output of Tx PA 220 and comprises a bandpassfilter 535, a delay circuit 540, and a VM 545 to generate a secondcancellation signal. The second cancellation signal is fed to an inputof the additive coupler 570 which adds the received signal with thesecond cancellation signal. The sum of the signals is amplified by RxLNA 235. The second pilot signal generation circuit comprises a bandpassfilter 504 coupled to the output of the LNA 235, a detector andcontroller unit 508, a limiter 510, a phase shifter 512, an absorptiveswitch 514, and a bandpass filter 516. The detector and controller unit508 may control the VM 545, the phase shifter 512, and the absorptiveswitch 514 through control lines 551, 552, and 553 respectively. Theoutput of the bandpass filter 516 is fed into additive coupler 518 whichadds the pilot signal with the signal generated by VVA 215.

FIG. 6 illustrates a system having a second cancellation circuit and amodified pilot signal generation circuit. The Tx signal path (i.e., thefirst signal path) comprises a WA 215 which receives the transmissionsignal 300 from the output of an amplifier 210, additive couplers 650and 670, a PA 220, and a Tx antenna 320. The Rx signal path (i.e., thesecond signal path) comprises an Rx antenna 360, additive couplers 370and 570, a LNA 235, and a VVA 240 which generates a received signal 385that is sent to the input of amplifier 245. The modified pilot signalgeneration circuit comprises a detector and controller unit 610, limiter615, and phase shifter 620. The output of the phase shifter 620 is fedinto absorptive switch 630 and 660. The output of the absorptiveswitches 630 and 660 are fed into filters 640 and 665 respectively. Thedetector and controller unit 610 may control VMs 345 and 545, the phaseshifter 620, and the absorptive switches 660 and 630 through controllines 651 a, 651 b, 652, 653 a, and 653 b respectively. The two pilotsignals are coupled to additive couplers 650 and 670.

In both examples, two Tx leakage cancellation circuits are providedhaving frequency responses H₁(ω) and H₂(ω), which are assumed to benon-overlapping in frequency. In the first case, two pilot signalgeneration circuits are provided that have the same frequency responsesas the cancellation circuits, H₁(ω) and H₂(ω). The two pilot signalgeneration circuits can be operated at the same time, assuming the PA220 and LNA 235 are linear. In the second case, only one pilot signalgeneration circuit is used. However, the filter response is switchedbetween H₁(ω) and H₂(ω) so that the most of the pilot signal generationcircuitry is multiplexed. Note that the filter before the detector isremoved.

The present invention has been described primarily as a system andmethod for cancelling transmission leakage signals in a wide bandwidthDistributed Antenna System (“DAS”) having remote units. In this regard,the system and methods for cancelling transmission leakage signals arepresented for purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Accordingly, variants and modifications consistent with thefollowing teachings, skill, and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedherein are further intended to explain modes known for practicing theinvention disclosed herewith and to enable others skilled in the art toutilize the invention in equivalent, or alternative embodiments and withvarious modifications considered necessary by the particularapplication(s) or use(s) of the present invention.

1-20. (canceled)
 21. An apparatus of a radio transceiver, the apparatus comprising: a transmission signal path coupled to a transmission antenna, the transmission signal path comprises a first coupler; a receiver signal path coupled to a receiving antenna, the receiver signal path comprises a second coupler; a cancellation circuit path that couples the transmission and receiver signal paths through the second coupler, the cancellation circuit path comprising a cancellation circuit, the cancellation circuit comprising a vector modulator (VM) configured to adjust an amplitude and phase of a cancellation signal supplied thereto to cancel Radio Frequency (RF) leakage from transmission signal path to the receiver signal path; and a pilot signal generation signal path that couples the transmission and receiver signal paths through the second coupler, the pilot signal generation signal path comprising a pilot signal generation circuit configured to generate a pilot signal and use the pilot signal to tune the cancellation circuit, the pilot signal generation circuit comprising a detector and controller unit having a VM control signal that controls the VM.
 22. The apparatus of claim 21, wherein: the pilot signal generation circuit is configured to tune the cancellation circuit by minimization of a power level of a residual pilot signal received from parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas.
 23. The apparatus of claim 21, wherein: the transmission signal path comprises a power amplifier coupled with the transmission antenna; and the receiver signal path comprises a low noise amplifier (LNA) and a variable voltage attenuator (VVA) coupled with the LNA, the coupler disposed between the receiving antenna and the LNA.
 24. The apparatus of claim 23, wherein: the transmission signal path further comprises an optical-to-RF converter and a transmission amplifier coupled with the optical-to-RF converter, the optical-to-RF converter configured to receive optical signals from a hub of a distributed antenna system (“DAS”), convert the optical signals to RF signals and supply the RF signals to the transmission antenna through the transmission amplifier and the power amplifier; and the receiver signal path further comprises an RF-to-optical converter and a receiver amplifier coupled with the RF-to-optical converter and with the VVA, the optical-to-RF converter configured to receive received RF signals from the VVA through the receiver amplifier, convert the received RF signals to received optical signals and supply the received optical signals to the hub.
 25. The apparatus of claim 21, wherein: the cancellation circuit further comprises a delay circuit, the delay configured to provide a delay matched to a network delay associated with a path formed by feedlines connected to the transmission and receiving antennas and RF coupling between the transmission and receiving antennas.
 26. The apparatus of claim 21, wherein: the pilot signal generation circuit further comprises: a generation bandpass filter, a limiter configured to limit, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit and parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, the limiter disposed between the detector and controller unit and the second coupler, a phase shifter, the phase shifter configured to adjust a phase of the feedback loop to zero, the phase shifter controlled by a phase control signal from the detector and controller unit, and an absorptive switch configured to open and close the pilot signal generation signal path.
 27. The apparatus of claim 26, wherein: the generation bandpass filter matches a cancellation bandpass filter that is connected with the VM in the cancellation circuit such that the VM is configured to adjust the amplitude and phase of the cancellation signal over a bandwidth that is narrower than a bandwidth of the apparatus.
 28. The apparatus of claim 26, wherein: the generation bandpass filter has a smaller bandwidth than a cancellation bandpass filter and that lies within a bandwidth of the cancellation bandpass filter, the cancellation bandpass filter connected with the VM in the cancellation circuit such that the VM is configured to adjust the amplitude and phase of the cancellation signal over a bandwidth that is narrower than a bandwidth of the apparatus.
 29. The apparatus of claim 26, wherein: the generation bandpass filter comprises: a first filter disposed between the detector and controller unit and the receiver signal path, the first filter configured to reduce a variance of a residual pilot signal received from parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, and a second filter disposed between the detector and controller unit and the second coupler, the second filter configured to remove harmonics of a frequency pilot of the pilot signal.
 30. The apparatus of claim 26, wherein: the absorptive switch is controlled by a switch control signal from the detector and controller unit.
 31. The apparatus of claim 26, wherein: the absorptive switch is controlled by an external signal controlling distributed antenna system (“DAS”) remote unit.
 32. The apparatus of claim 26, wherein: the absorptive switch is configured to disable the pilot signal generation circuit when the apparatus operates as part of a distributed antenna system (“DAS”).
 33. The apparatus of claim 26, wherein: the transmission signal path comprises a power amplifier coupled with the transmission antenna; the receiver signal path comprises a low noise amplifier (LNA) and a variable voltage attenuator (VVA) coupled with the LNA, the coupler disposed between the receiving antenna and the LNA; and the VVA is set to a maximum attenuation during calibration.
 34. The apparatus of claim 31, wherein: the cancellation circuit path comprises a plurality of the cancellation circuits; and the pilot signal generation signal path comprises a plurality of the pilot signal generation circuits.
 35. The apparatus of claim 34, wherein: each cancellation circuit comprises a cancellation bandpass filter covering a different frequency range
 36. The apparatus of claim 34, wherein: the pilot signal generation circuits comprise a first and a second pilot signal generation circuit, each of the first and second pilot signal generation circuit comprising: a detector and controller unit having a VM control signal that controls a VM of a different cancellation circuit, a generation bandpass filter, a limiter configured to limit, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit and parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, the limiter disposed between the detector and controller unit and the transmission signal path, a phase shifter configured to adjust a phase of the feedback loop to zero, the phase shifter control led by a phase control signal from the detector and controller unit, an absorptive switch configured to open and close a pilot signal generation signal path associated with the respective pilot signal generation circuit, and a bandpass filter disposed between the respective detector and controller unit and the transmission signal path, the bandpass filters of the first and second pilot signal generation circuits having non-overlapping frequency responses.
 37. The apparatus of claim 34, wherein: the pilot signal generation circuits comprise: a detector and controller unit having a first VM control signal that controls a VM of a first cancellation circuit and a second VM control signal that controls a VM of a second cancellation circuit, a limiter configured to limit, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit and parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, the limiter disposed between the detector and controller unit and the transmission signal path, a phase shifter configured to adjust a phase of the feedback loop to zero, the phase shifter controlled by a phase control signal from the detector and controller unit, first and second generation bandpass filters each disposed between the detector and controller unit and the transmission signal path, the first and second bandpass filters having non-overlapping frequency responses, and first and second absorptive switches each configured to open and close a pilot signal generation signal path associated with the first and second generation bandpass filter respectively.
 38. The apparatus of claim 21, wherein: the apparatus is a remote unit of a distributed antenna system (“DAS”).
 39. A method of reducing interference of a transmission signal path in a receiver signal path of a radio transceiver, the method comprising: coupling the transmission and receiver signal paths through a pilot signal generation signal path and a cancellation circuit path; generating a pilot signal in the pilot signal generation signal path; launching the pilot signal through the transmission signal path and from a transmission antenna; receiving the pilot signal through Radio Frequency (RF) reception at a receiving antenna; generating a vector modulator (VM) control signal based on a residual pilot signal received from parallel paths of the cancellation circuit path and RE coupling between the transmission and receiving antennas; and adjusting, using the VM control signal, an amplitude and phase of a cancellation signal to minimize a power level of the residual pilot signal and cancel leakage from transmission signal path to the receiver signal path.
 40. The method of claim 39, further comprising: delaying the pilot signal through the cancellation circuit path to match to a network delay associated with a path formed by feedlines connected to the transmission and receiving antennas and RF coupling between the transmission and receiving antennas.
 41. The method of claim 39, further comprising: bandpass filtering the pilot signal in the pilot signal generation signal path; limiting, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit path and parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, adjusting a phase of the feedback loop to zero, and controlling the pilot signal generation signal path to open and close the pilot signal generation signal path.
 42. The method of claim 41, further comprising: bandpass filtering in the pilot signal generation signal path using a smaller bandwidth than in the cancellation circuit path.
 43. The method of claim 41, further comprising: providing a plurality of parallel pilot signal generation signal paths and a plurality of parallel cancellation circuit paths in which the parallel cancellation circuit paths are bandpass filtered using non-overlapping frequency ranges and each pilot signal generation signal path is associated with a non-overlapping frequency range of a different cancellation circuit path.
 44. The method of claim 43, further comprising in each of the parallel pilot signal generation signal paths: bandpass filtering the pilot signal, limiting, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit path and parallel paths of the cancellation circuit path and RF coupling between the transmission and receiving antennas, adjusting a phase of the feedback loop to zero, and controlling the pilot signal generation signal path to open and close the pilot signal generation signal path.
 45. The method of claim 43, wherein: in each of the parallel pilot signal generation signal paths: bandpass filtering the pilot signal, and controlling the pilot signal generation signal path to open and close the pilot signal generation signal path, and controlling a VM of each cancellation circuit using a common detector and controller unit associated with the parallel pilot signal generation signal paths, limiting, to at most unity, a loop gain of a feedback loop comprising the pilot signal generation circuit paths and a combination of the parallel cancellation circuit paths and RF coupling between the transmission and receiving antennas, the limiting performed using a common limiter associated with the parallel pilot signal generation signal paths, and controlling, by the common detector and controller unit, a common phase shifter to adjust a phase of the feedback loop to zero. 